For the hot sections of gas turbine engines, three types of so-called “superalloys” are used: solid solution-strengthened nickel alloys, precipitation-hardenable nickel alloys, and solid solution-strengthened cobalt alloys. All of these alloys contain chromium (usually in the range 15 to 30 wt. %), which imparts oxidation resistance. The precipitation-hardenable nickel alloys include one or more of aluminum, titanium, and niobium, to induce the formation of very fine gamma-prime (Ni3Al,Ti) or gamma-double prime (Ni3Nb) precipitates in the microstructure, during aging.
The precipitation-hardenable nickel alloys have two drawbacks. First, they are prone to problems during welding, since the heat of welding can induce the formation of hardening precipitates in heat-affected zones. Second, the gamma-prime and gamma-double prime precipitates are only useful to certain temperatures, beyond which they coarsen, resulting in considerably reduced material strengths. The solid solution-strengthened nickel and cobalt alloys, on the other hand, lack the strength of the precipitation-hardenable nickel alloys, but maintain reasonable strengths at higher temperatures, especially those based on the element cobalt.
Unlike nickel, which has a face-centered cubic (fcc) structure at all temperatures in the solid form, cobalt exists in two forms. At temperatures up to about 420° C., the stable structure is hexagonal close-packed (hcp). Beyond this temperature, up to the melting point, the structure is fcc. This two-phase characteristic is also shared by many cobalt alloys. However, the alloying elements shift the transformation temperature up or down. Elements such as iron, nickel, and carbon are known stabilizers of the fcc form of cobalt and therefore reduce the transformation temperature. Chromium, molybdenum, and tungsten, on the other hand, are stabilizers of the hcp form of cobalt and therefore increase the transformation temperature. These facts are important because they strongly influence the mechanical properties of the cobalt alloys at ambient temperatures.
The reason is that the fcc to hcp transformation in cobalt alloys is sluggish, and, even if the transformation temperature is above ambient, the hcp form is difficult to generate upon cooling. Thus many cobalt alloys possess metastable fcc structures at room temperature. Conversely, the hcp form is easily generated during cold work, the driving force and extent of transformation being related to the transformation temperature. Those metastable cobalt alloys with high transformation temperatures are, for example, difficult to cold work and exhibit high work hardening rates, due to the formation of numerous hcp platelets in their microstructures. Those metastable cobalt alloys with low transformation temperatures are less difficult to cold work and exhibit much lower work hardening rates.
One of the requirements of wrought, solid solution strengthened cobalt alloys used in gas turbines is that they be capable of at least 30% cold reduction, so that sheets of fine grain size might be produced. Thus, nickel is normally included in such materials, to reduce their transformation temperatures, and in turn to reduce their tendency to transform during cold rolling.
Attempts to use the precipitation of intermetallics (such as gamma-prime) to strengthen cobalt alloys have foundered (equivalent cobalt-rich intermetallics have lower solvus temperatures than gamma-prime). However, an alternate method of strengthening cobalt alloys was disclosed by Hartline and Kindlimann in U.S. Pat. No. 4,043,839. But, this method is useful only for thicknesses regarded as impractical for the construction of gas turbine components (less than 0.025″, and preferably less than 0.01″). Their method involved a procedure for absorbing and diffusing nitrogen into cobalt alloys, to induce the formation of a fine dispersion of nitride particles. According to Hartline and Kindlimann, alloys that respond to such treatment contain at least 33% cobalt as the major constituent, chromium, up to 25% nickel, up to 0.15% carbon, and 1 to 3% of nitride forming elements from the group consisting of titanium, vanadium, niobium, and tantalum. Residuals and elements which enhance the properties of cobalt-base alloys, notably molybdenum and boron, were also mentioned. No mention was made of iron, although iron was present at the 1% level in samples successfully nitrided by these inventors. A sample containing 29% nickel, which was less amenable to nitridation, contained 2.7% iron.